辽西医巫闾山变质核杂岩西北缘韧性变形特征及构造意义

Ductile deformation characteristics and tectonic significance of northwestern Yiwulüshan metamorphic core complex in western Liaoning

辽西医巫闾山变质核杂岩记录了中国东部中生代克拉通减薄或燕山运动体制下的重要变质变形过程。为了进一步探究其变形样式和地球动力学背景,笔者对医巫闾山变质核杂岩西北缘发育的韧性剪切带进行了详细的野外地质调查、微观构造解析和石英EBSD组构分析。清晰地识别出两期韧性剪切变形事件,早期表现为右行韧性剪切变形,兼具水平走滑分量,石英和长石分别以GBM、BLG为主要显微变形机制,石英EBSD组构指示柱面滑移为主导滑移系,变形温度约为550~650℃;晚期为左行伸展剪切变形,石英颗粒以SGR为主要变形机制,EBSD组构以菱面滑移为主导滑移系,变形温度约为400~500℃,同时可识别出两期变形叠加的特点。强变形长英质糜棱岩的锆石U-Pb年代学测试显示其结晶年龄为(157.4±1.6)Ma,代表区域晚侏罗世构造热事件。综合前人研究成果,将两期韧性剪切变形时代限定于中晚侏罗世和早白垩世,分别与燕山运动多板块汇聚体制下伊泽那崎板块向欧亚板块俯冲和后撤有关。

The Yiwulüshan metamorphic core complex in western Liaoning records important metamorphic and deformation processes under the Mesozoic craton thinning or Yanshan Movement regime in eastern China.In order to further explore its deformation characteristics and geodynamic background,the ductile shear zones developed in the northwestern Yiwulüshan metamorphic core complex were investigated by detailed field investigation,microscopic deformation analysis and quartz EBSD fabric analysis in this paper.Two ductile shearing events were clearly identified.The early deformation is characterized by dextral shearing with horizontal strike-slip,and quartz and feldspar take GBM and BLG as the main microscopic deformation mechanisms,respectively,and quartz EBSD fabric takes prismaticslip as the dominant slip system,with the deformation temperature of about 550~650℃.The later deformation is sinistral extensional shearing deformation,SGR is the main deformation mechanism of quartz grains,and their EBSD fabrics show rhombslip as the dominant slip system,with the deformation temperature of about 400~500℃,and the superposition of two phases of deformation can be identified.Zircon U-Pb dating from felsic mylonite shows that its crystallization age is(157.4±1.6)Ma,representing the regional Late Jurassic tectonic thermal event.Based on previous research results,the ages of two ductile deformation events are limited to the Mid-Late Jurassic and Early Cretaceous,respectively,which are related to the subduction and roll-back of the Izanagi Plate to the Eurasia Plate under the multi-plate convergence regime of the Yanshan Movement.